The present invention relates to data transfer from parallel transfer disk drives and more particularly to a method of and apparatus for reading data from parallel transfer disk drives notwithstanding a fault occurring in a data transmission channel.
Parallel transfer disk drives comprise a plurality of disk platters stacked to rotate as a single unit on a spindle. Each platter has top and bottom surfaces covered with a magnetically polarizable media. A magnetic transducer associated with each magnetic media surface is used to write and read data records (i.e. generate and detect associated groups of magnetically polarized regions) on the surfaces.
The transducer and the magnetic media for each platter present to one another surfaces that are in essentially constant motion with respect to one another. While the interaction between transducer and media is magnetic, the interaction between their surfaces is a mechanical one, affected over time by factors such as friction, wear, media oxide flaking and collisions between the transducer and the media. In addition, both individual transducers and magnetic media surfaces are subject to failures, such as opening of a transducer electromagnet winding. Such mechanical changes affect the magnetic interaction of the transducer and the media, and consequently can affect the ability of data recovery circuitry to read data records from the media surface. All of these faults can result in partial loss of access to data records from the affected disk drive.
Data recovery circuitry is associated with each magnetic transducer for reading the records. While the operation of such circuitry is not strictly related to the invention of this patent, knowledge of certain characteristics of the interaction of these circuits with the data records is useful to understanding the invention.
Intelligence, in data records, is associated with the relative timings between transitions in the polarization states of sets of magnetically polarized regions on a magnetic media surface. To the data recovery circuitry, these transitions appear as transitions in the voltage level of an electrical signal. At least in theory, the meaning of the data record will remain the same regardless of the speed at which the platter it is located on is moving relative to the transducer, so long as the speed is constant. The clock of a data record is recovered from the data record itself. Accordingly, each data record includes a field near its beginning from which the clock of the signal is established. If the data recovery circuitry can recover the clock, recovery of the data can proceed. Where the data recovery circuitry fails to recover the clock, probably as a result of one of the faults described above, data recovery on that channel cannot successfully proceed.
Parallel transfer disk drives achieve high rates of data transfer by the simultaneous transfer of data bytes over a plurality of channels to and from the plurality of available magnetic media surfaces. In conventional drives, data is recovered serially. The magnetic transducers are supported by a carriage providing for movement of the transducers in unison. Data records are positioned in a plurality of concentric circular tracks, which are located on each magnetic media surface. The carriage positions the magnetic transducing heads over the same concentric track on each platter, allowing reading and writing of data records at the same address on each platter at substantially the same instant.
Current multiplatter disk drives abort a data recovery operation whenever one of the channels fail in a data recovery attempt. Failures of multiplatter disk drives are more common than failures of single platter disk drives as a result of the increased number of transducers and magnetic media surfaces which are interacting.
Avoidance of an abort in a data read resulting from loss of a channel requires, at a minimum, a data recovery system allowing regeneration of the data of the lost channel. Otherwise, continuation of the read would be futile. Schemes for data recovery systems applicable to parallel transfer drives exist. Data in digital processing systems is typically organized into groups of adjacent bits acted upon by system as a unit. Such units (or "bytes" as they are commonly called) are stored to the disk drive by assigning each bit of the byte to the same logical address on different platters (different transducers). In other words, each logically associated byte is divided among a plurality of physically associated data records. Allowance for loss of the data from a record can be provided through use of redundant data for regeneration of the record.
Examples of redundant data include parity data and error correction code syndromes generated from a logically associated group of data bits, such as a byte. The redundant data for a byte can be generated upon transfer of the byte to the disk drive for storage. The redundant data can then be stored in some fashion which preserves its relationship with the data group for which it was generated.